Element disappears when attempting drop away from the drop target

Lior

I wrote a little demo that has two divs, one contains draggable buttons and the other is the drop target.
I can drag buttons into the drop target just fine, and the drop target itself is also draggable so I can move the buttons around inside the drop target.

This works, but when I try to drag a button from the dropTarget div to some other place on the page, instead of just failing to drop it there (on a non-droppable section), the button disappears from the drop target div.

This is the code, I'd appreciate if someone could tell me why this happens and if it can be avoided:

Progress, Telerik, and certain product names used herein are trademarks or registered trademarks of Progress Software Corporation and/or one of its subsidiaries or affiliates in the U.S. and/or other countries. See Trademarks or appropriate markings.